1. Field of the Invention
This invention relates to methods of coating substrates with metals and devices made using the methods of the invention.
2. Description of Related Art
Thin coatings of metals are vitally important in many industries. The microelectronics industry, for example needs thin coatings of metals to make integrated circuits, where size of the device is critical to performance. In catalysis, metals play an important function in automobile exhaust catalysis and in many other reactions. Since some catalysts, such as platinum, can be extremely expensive, a thin coating of the catalytic metal on the surface of the catalytic body is desirable.
In many applications of thin metal films, selective coating is required. For example in the microelectronics industry, a metal, such as platinum, must be selectively coated on a semiconductor surface to provide interconnects to the various circuit elements. If the wrong circuit elements are connected, then the whole semiconductor chip may become worthless.
In some applications of metal coating, high purity of the deposited metal is required. For example in catalysis, the metal catalyst can be poisoned by impurities. And, in the microelectronics industry, impurities can damage a circuit element or the function of an interconnecting link between the various circuit elements.
In some applications, a thorough coating of the metal is needed. For example in the microelectronics industry, if a metal is designed to separate two other layers from each other, a less than complete coating could damage the circuit or cause a short-circuit.
Several methods have been proposed for placing a thin coating of metal on a substrate, including precipitation from liquid solution, sputtering, and chemical vapor deposition (CVD). Liquid solution precipitation does not always provide selective deposition or the proper degree of purity, and sputtering does not always provide a thorough coating, since it is restricted to line-of-sight deposition and may miss nooks and crannies in the substrate. In addition, sputtering often requires a mask to achieve selective deposition, or the extra metal must be etched away in a subsequent treatment, which results in loss of a potentially expensive metal.
CVD, on the other hand, can provide selective deposition under the right circumstances. Moreover, since it does not rely on line-of-sight deposition techniques, it will provide a thorough coating on a substrate containing nooks and crannies. CVD, however, suffers from problems with obtaining satisfactory purity levels in the metal. Chemical vapor deposition involves decomposing a volatile compound containing a metal on a surface that is typically heated. There are at least three ways to accomplish the decomposition of the volatile metal compound: reduction, thermal decomposition and displacement. Groshart, E., "Metalizing Nonconductors," Metal Finishing, p.49 (August 1972).
In reduction, hydrogen or some other reducing gas is exposed to the volatile metal compound either during or after deposition on the substrate. The hydrogen reacts with the non-metal portion of the compound, leaving the metal behind on the substrate. In thermal decomposition, the substrate is heated, and the volatile compound reacts with itself to form a nonmetallic material that leaves the substrate and a metal that stays. In displacement, a material already present on the surface of the substrate reacts with the volatile compound and "changes place" with the metal to be deposited.
In each of these techniques, however, the purity of the deposited metal can be affected by the other components of the volatile compound and by any contaminant in the system. For example when platinum acetylacetonate, Pt(CO).sub.2 (Cl.sub.2), and platinum trifluorophosphine were tried on a substrate-containing silicon, each platinum coating showed signs of contamination by other elements. Rand, M., "Chemical Vapor Deposition of Thin-Film Platinum," J. Electrochem. Soc. Solid-State Science and Technology, 120, 686 (1973).
One potential source of volatile compounds for thin metal films for CVD is organometallics. These compounds provide high growth rates, ease of process control and generally higher purity than some other volatile compounds. See Gozum, J. et al., "`Tailored` Organometallics as Precursors for the Chemical Vapor Deposition of High-Purity Palladium and Platinum Thin Films," J. Am. Chem. Soc., 110, 2688 (1988). In that article, bis(allyl)palladium, bis(2-methylallyl) palladium and (cyclopentadienyl)(allyl)palladium were reported to be tested for CVD at 250.degree. C. and 0.0001 torr. The article reported that the cyclopentadienyl compound had about 5% residual carbon as a contaminant. The article also reported that the cyclopentadienyl complex CpPtMe.sub.3 [(cyclopentadienyl)(trimethyl)platinum] "yield[s] high-quality Pt films under similarly mild (250.degree. C., 10.sup.-4 torr) conditions." That article, however, did not report on the necessity of a reducing gas, especially hydrogen, to obtain high purity. Titanium carbide (TiC) films have also been deposited using tetraneopentyltitanium (Ti[CH.sub.2 C(CH.sub.3).sub.3 ].sub.4) at approximately 150.degree. C. There was enough carbon present after CVD to account for a separate TiC phase in the deposited material.
Organometallic compounds have also been evaluated for other purposes. For example, in Egger, K., "Cyclopentadienyl-Metal Complexes II. Mass Spectrometric and Gas Phase Kinetic Studies on the Thermal Stability and the Structure of (CH.sub.3).sub.3 Pt-C.sub.5 H.sub.5," J. Organometallic Chemistry, 24, 501 (1970), testing to determine the structure of cyclopentadienyltrimethylplatinum(iv) was reported. The article concluded that the cyclopentadienyl group was Pi bonded. Analysis of the hydrogenation of (1,5 cyclooctadiene) dimethylplatinum has also been reported. Miller, T. et al., "Heterogeneous, Platinum-Catalyzed Hydrogenation of (Diolefin)dialkylplatinum(II) Complexes: Kinetics," J. Am. Chem. Soc., 3146 (1988) (and two subsequent articles by the same author). Hydrogenation of nickelocene ((C.sub.5 H.sub.5).sub.2 Ni) has also been reported. Kaplin, Y. et al., "Decomposition of Nickelocene in Presence of Hydrogen," UDC 547.1'174, c. 1980 Plenum Publishing Corp., translated from Zhurnal Obshchei Khimii, 50, 118 (1980). None of these articles, however, report any special benefits of using the compounds in CVD.
Thus, there is presently a need in the art for an organometallic compound useful in CVD of metals that does not contaminate the deposited metal with impurities and that uses a reducing gas, such as hydrogen, to remove the organic ligands during or after deposition.